Which octane?

I have 2 cars (non turbo) that both “recommend” high octane fuel (91 or higher). With the price of gas now I’ve been using regular gas (87).

Both cars seem to handle it fine. There’s no knocking or noticeable lose of power, but will it harm anything by doing this?

You don’t say what’s your vehicle make, model and year. Most cars made after the early 1990’s have anti-knock sensors and will adjust the timing to compensate for different fuel octane. If your manual says XX octane REQUIRED, then use it. If it says YY octane RECOMMENDED then any grade will do. Lower octane will give less power, but will not harm your engine. Use the forum search function – this question has been asked many times over.

Probably not, but there’s a fair chance you’re taking a hit in fuel economy. The amount varies from car to car, with some cars it’s not noticeable, on some cars, like Nissans equipped with the pre-2006 VQ V6, it’s very notciable. So you may or may not be actually saving any money.

What cars are we talking about here? If it’s an older continental with the 3.8L you probably won’t notice anything. If it’s a newer one with the DOHC 4.6L you might lose a bit of mileage, but nothing drastic. You probably won’t hurt anything, and you don’t have any warranty issues to worry about as this point, so you’re probably okay using regular if you want. If it were me, I just use whatever the manual recommends though. In the grand scheme of car ownership the cost between regular and premium fuel is almost trival.

If anything damaging or destructive is going on, you WILL be able to detect it (by sound) and switch back to so-called premium fuel before to hurt anything…

It is my personnel opinion that premium fuel exists because it allows consumers to screw themselves to the tune of .20-.30 cents a gallon and it increases the “pool margin”, the overall profit margin enjoyed by the oil companies who are generating profits never before imagined by any corporation…I also suspect that the oil companies pay dearly to have the sentence “Premium Fuel Recommended” placed in owners manuals…They pay even more for the words “Premium Fuel Required”…

The recommended octane will likely be the most economical in the long run. Using too low an octane is all about less efficiency at the engine. You likely are getting lower fuel mileage and may be spending more money than you would if you used the higher octane. You also are likely getting less power from your engine.

Premium fuel exists because some cars require it. What consumers who do not own such cars do with regard to buying premium is not the oil company’s problem.

As long as the engine isn’t knocking…then there should be no damage.

HOWEVER…vehicles are equipped with anti-knock sensors to prevent knocking. It does this by retarding the timing…thus giving you lower performance and decreased gas mileage.

It makes no sense to buy regular if your car requires hi-test…and visa-versa.

While browsing TSB’s the other day I found one from Chrysler that linked drivability issues with the use of fuel with a high "drivability index’. High octane fuels have a high drivability index. The TSB ended saying “dont use 91 octane in a car designed for 87 octane”. I admit this was a first as before I thought the only result of using higher than required octane fuel was simly wasting money.

Then there is the GM TSB linking valve stem siezure when a steady diet of non-Top-Tier fuel is used.

Wonder what high octane fuel would do? How would it cause a problem?

I posted this a few days ago,let me see if I can find it. Make that “low drivability index”,high octane from the FAQ site

Here is a link,there is a lot out there from Chrysler on this. One TSB is 113613


It’s more resistant to burn, which means some of it doesn’t burn completely. Over time that results in carbon deposits within the cylinder which heat up as the motor runs. If you switch to regular gas again, the hot carbon then preignites the fuel/air mix before it’s supposed to be ignited, also known as predetonation. At that point you either have a costly job getting the carbon out of your cylinders, or you have to run premium because you’ve made the motor run poorly on regular.